This is a new Project. Normal appearing human bronchial explants were exposed to chemical and physical carcinogens. Examination of the carcinogen-treated explants by light and electron microscopy revealed marked cellular atypia in the non-cornifying epidermoid metaplastic epithelium and in the exfoliated epithelial cells. In addition, autoradiograms showed an increase in labeling index with 3H-Thymidine. The tumorigenic potential of these carcinogen-induced preneoplastic lesions is being studied in athymic nude mice.